When a liquid is placed on a solid surface, the liquid surface assumes a shape that is characteristic of the physiochemical properties of the three phases involved (solid, liquid and vapor phase). The angle defined by the liquid and solid surfaces at the point of contact of the three phases is termed the “contact angle” (θ). The magnitude of this angle is determined by the interfacial free energies (surface tension, γ) of the liquid-vapor (LV) interface, the liquid-solid (LS) interface, and the solid-vapor (SV) interface. In the case of liquids placed within a dish or well of a multiwell plate, a meniscus results when the contact angle between the liquid and the solid surface is other than 90 degrees. When the contact angle is less than 90 degrees, a concave meniscus is formed, and when the contact angle is greater than 90 degrees, a convex meniscus is formed.
Due to the contact angle properties defined above, surface energies of the solution and the containing solid interfaces are often cited as defining properties that determine meniscus shape and magnitude. However, physiochemical properties, in addition to surface energies of the liquid and solid surfaces, are of importance in determining meniscus shape of aqueous liquids at equilibrium. Such properties include (a) the three-dimensional topology of the solid surface, (b) the composition of the liquid phase, (c) physical and chemical heterogeneity of the solid surface, and (d) inducibility of configurational changes of the solid surface by the liquid. This causes a hysteresis in contact angles dependent on the interactions of the above mentioned surface properties, which makes the theoretical estimation of meniscus magnitude difficult based on surface chemistries. There is currently no universal theory which accurately models the contact angle in complex systems, and thus contact angle hysteresis and the resulting meniscus must be determined empirically for different liquid/solid combinations.
A meniscus presents a significant problem to any optical imaging of the objects present within a liquid containing vessel, dish or well for several reasons. The curvature of the liquid surface will cause the refraction and reflection of the illuminating light and result in optical interference within the area of the meniscus. In addition, the meniscus results in a change in liquid depth near the solid surfaces and may cause an uneven distribution of objects near the wall of the dish or well.
Several methods have been used to compensate for meniscus effects in digital microscopy. Observations can be limited to central portions of a well or the light exposure can be increased when imaging within the meniscus. Physical barriers, such as coverslips, have also been used to compress the meniscus. However, such methods are cumbersome and can reduce the sampled image area so as to omit relevant areas of cell cultures and render the method not quantitative.
One technique for mitigating meniscus formation is to compose the solid surface of the vessel wall from a polymeric material that provides surface properties at the liquid-solid interface between a particular polymeric material and a particular liquid so as to result in a dynamic minimum contact angle of approximately 90 degrees. Alternately, a polymeric coating may be applied to the wall surface to impart those same properties to the surface. However, since these surface properties of the liquid-solid interface are dependent on the complex interactions between physiochemical properties of the liquid as well as the solid surface chemical and physical heterogeneity, different wall polymers or surface coatings would be required for liquids with dissimilar properties.
A uniform method that is effective for reducing meniscus magnitude with a wide variety of aqueous solutions with various surface energies would be advantageous for many imaging and biological applications.